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Difficulty Starting Engine When Warm...

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For a while now, I've been noticing that my car is getting more and more difficult to start when warm/hot (not overheating, just at operating temp).  When cold, the car will usually turn over instantly.  When it is hot though, it requires LOTS of cranking, holding the gas to the floor, and prayers to get the car started.  I just put in an s14 starter, which runs strong, so that isn't the issue (also, battery is only a year old).  Instead of firing up, the car sort of coughs it's way to life if it isn't cold.  (sounds weak, like it's misfiring or something, till I get it turned over, and idling).  I have had 0 issues once the engine is started.  It accelerates well (though it is smokey), and never stalls at idle.  


Yesterday, after ~5 hours of driving (at the Chicago 2002/356 cruise), I stopped at a gas station, and was stranded for 30 minutes.  Starting fluid had no effect.  I wasn't even getting combustion pops.  It took ~30 tries of cranking (with varying amounts of waiting in between) to get it running.  Finally, it limped itself on, and started idling.  


What would cause this?  My intuition tells me that the compression is shot and the car needs a rebuild.  I just ordered new spark plugs in case that is causing this.  What do you guys think?

1976 Fjord Blue BMW 2002

2002 Topaz Blue BMW M3 Convertible

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You say the car smokes on acceleration....how about on startup, and on the overrunn (foot off the gas and letting it coast in gear.)  Generally speaking...smoke on acceleration = worn rings; smoke on the overrun = worn valve guides/stem seals.  In both cases, low compresson


Have you done a compression test on the car?  Try it wet and dry; if there's a significant difference  after a squirt of oil into the cylinder ("wet")then you're losing compression through worn rings.  That can cause hard starting.  If there's more than 10% difference between the best and worst compression in the cylinders, it's getting to be time for some internal engine work.  You can also get a good idea of compression with a leakdown test, but not as many people have the equipment to do one, more of us have compression gauges. 


If the compression checks out OK, you might suspect fuel boiling in the fuel line somewhere in the engine compartment.  If the fuel line that runs from the firewall to the fuel pump (and thru an in-line filter if you have one) is touching the cylinder head, for example, you can get boiling in the line, and the fuel pump won't suck fumes.  So make sure your fuel line is suspended away from the head, isn't porous and that the fuel filter is not clogged.  To rule this out as a problem, wrap the fuel line in aluminum foil and see if that helps. 


One other thing--is your carb sitting on an insulating block (piece of 3/16" or so bakelite) that is sandwiched between carb and manifold?  They are used to keep the heat in the manifold from soaking the carb and causing fuel to boil in the float chamber--another cause of hard hot starting.


Let us know what you discover so we can further long-distance diagnose.




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Mechanical fuel pump?


My first guess is that your float valve's leaking, and it floods the carb when you switch off.

But that's just a guess.


Other things (weak spark, vacuum leaks at the carb base, dying condeser, plug wires, etc) can do it, too.



"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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A couple details from the drive that may help... I was riding with Mike on the first leg of the trip. The car was a bit difficult to start after it sat in the lot after arriving at our starting point, about 15-20 minutes, and when we stopped for a few minutes to let folks catch up, it started up right away. Time shut down also seems to be a factor.

Come over and we'll put in that Korman stroker and call it a day!

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-tank pick up strainer ?


-does this happen more so with low tank level?

never when full ?


-old rubber fuel hoses will suck air but not leak any

gas visibly - replace all rubber gas hoses  - rear and in engine



-mechanical fuel pump diaphram dead or near dead - replace the pump

if 30 + years old


-issue with rubber tipped float needle and seat.


-Float level needs checking


-the brass fuel fitting - the 'seat' can raise up out of the

aluminum carb body causing a erronious fuel level - with

carb top off - gently take a brass drift and try tapping the brass seat

fully home. recheck/adjust the float level


of course compression test, valve adjustment, and ignition timing

have somme effect of starting and running anytime.....

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
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Looks like I have my fair share of homework...


First, notes:

  • Smoke is on acceleration only
  • Starting difficulty is unrelated to gas level
  • Fuel filter has gas in it at all times, at both sides of filter
  • Gasoline smells in engine bay and trunk, unsure of location or cause
  • I have a sweet stroker motor sitting in the garage  ;)

My plan of action:

  • Compression test
  • Replace gas lines - how many feet/meters needed? (currently have 1m)
  • Research and diagnose float level issue
  • Research and diagnose vacuum leaks
  • Research and diagnose mech. fuel pump


    say fuck it and swap in the stroker motor


Sound good?  Should I reorder my action list?

1976 Fjord Blue BMW 2002

2002 Topaz Blue BMW M3 Convertible

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Gas smells.... reminds me of my 3.0CS...."they all do that".


There are numerous possibilities.  First and foremost you must check for visible leaks when the car's running.  Immediately fix any visible leaks.  Gas and cars burn really quickly.   DAMHIK.


Trunk gas smells can be the flex hoses from the sender are shot, replace them, only about 2 feet.  Next with a full tank, forcefully shake the car side to side and see if there is any weeping near the o-ring at the sender.  Although I've read the o-ring will shrink when dried a day or two, I've also read that it's non-reusable and should only be replaced.


Trunk smells can come from bad hoses to/from the white colored evaporative emissions control canister that sits above the right inner fender.  Check and replace as/if needed.  Also see where the vent line ends up under the hood.


Trunk smells can come from the vent line which should be connected to the rubber filler elbow.  Verify the plastic line is intact and that the filler elbow is not cracked.


Passenger compartment gas smells come from both front and back.  Ifyour trunk lid gasket isn't sealing and there are fumes in the trunk for any reason, you'll smell them in the driver's seat.  Climb in the trunk and look for daylight along the seal, or put a piece of paper over the seal and close the lid.  The paper should drag noticeably when pulled out, indicating a good seal fit.  If the car has been rear-ended, the entire back section may be slightly bent, causing loss of seal.  The gas tank may have a pinhole leak along the seal between the two halves.  To assess this you can look for a wet spot from above with the floorboard out, or below just visually assess.  Usually you have to remove the tank to really check this one - do it when the tank is nearly empty, then thoroughly clean the tank and body support flanges; replace the foam seal with more modern material that does not absorb and retain water.


Engine compartment smells come into the passenger compartment through the ventilation system, e.g. if your wiper gutter no longer has the tubular rubber seals, they'll let fumes in via the vent system.


Jeez, I could go on but this will give you something to do this afternoon.

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